Solvent extractants are well known for the recovery of metals, especially copper, from aqueous streams and include oxime reagents, particularly o-hydroxyarylaldoximes and o-hydroxyarlyketoximes. While such reagents have been found to work well in the recovery of copper from solution, one problem that has been encountered in the application of such reagents is that the aldoxime and ketoxime reagents which are in continual use in metal extraction circuits can be degraded and the level of degradation can impair the efficiency of metal transfer from leach solution to strip solution. In particular it has been noted that this problem is more pronounced when metal values are extracted from certain ores. In Chile there are large mineral deposits of what are referred to as porphyry copper deposits. Normally such deposits do not contain high nitrate concentrations, but in very few parts of the world the deposits may contain large amounts of nitrate. Of particular note are the copper deposits in the Atacama Desert region of northern Chile, where mineralogy of the region gives rise to nitratine or Chile saltpetre (NaNO3), a mineral with high solubility as well as nitre (KNO3). These ores have been found to cause problems in the solvent extraction process especially with the degradation of solvent extraction reagents.
In recent years a number of articles and patents have appeared which have offered ways to reduce the rate of degradation of oximes in contact with nitrate containing feeds. Many of these involved operational adjustments to the pregnant leach solutions (PLS). These included reducing the nitrate concentration, raising the pH, controlling the redox potential of the feed, and reducing the transfer of impurities from the leach solution to the electrolyte. Given the high flow rates of the PLS in the solvent extraction processes, combining one or more of these adjustments into a extraction circuit flow sheet is not a trivial matter. Addition of nitrous acid quenchers such as sulphamic acid or urea to the circulating electrolyte is shown to be an effective protection for the oximes if nitrate ions are transferred from the leach to electrolyte solution. Another way to address the problem was to use a reagent based on 2-hydroxy-5-alkyl-acetophenone oxime (ketoxime) which has been reported to show increased stability to that of salicylaldoximes in contact with aqueous streams of this type. Ketoxime formulations do contain a higher concentration of residual nonyl phenol carried through from their manufacture and, as shown in this application for patent, this residual phenol could, in part, be responsible for their increased stability over that of salicylaldoximes. Ketoximes are weaker reagents than salicylaldoximes so in using these o-hydroxyarylketoximes the copper extraction could be limited when moving to higher tenor or lower pH feeds. Providing a strong oxime formulation that is stable to these aggressive aqueous feeds would negate the need for costly adjustments to the PLS whilst enabling the maximium copper recovery from high copper/low pH solutions.
Reagent formulations which could resist this enhanced degradation in contact with these streams or similar conditions would be advancement on existing technology.